(c) above defines forward pistol grip as allowing a pistol style grasp forward of the trigger. It says NOTHING that includes the definition of a "pistol grip that protrudes conspicuously beneath the action of the weapon" from part (d).

So there's a disconnect. While it's logical that (c) would include (d), it doesn't. So a forward pistol grip is not bound by the web of the hand below the action definition.

(c) just states if you can get a pistol style grasp on a GRIP mounted in front of the trigger, then it's a forward pistol grip. It specifies "grip", so things like sling mounts, bipods, light mounts are not included.